The invention disclosed relates to an assemblage of components for the drive system of the worktable in an automatic gear cutting machine.
A fundamental requirement exists in gear cutting machines for synchronization between the movement of the rotatiny tool, or hob, and that of the work, to the end of ensuring precise geometry and spacing of the teeth in relation to the pitch diameter of the formed gear, whatever the type of tooth -viz, spur or helical, relieved, crowned, etc.
In conventional machines, synchronized movement of this nature is achieved by way of a mechanical linkage that connects the tool spindle and the table carrying the work.
Machine tools of the type in question generally incorporate a column that carries the tool spindle and can be moved toward and away from the worktable. The tool, which is generally a straight (or taper) hob, is carried in such a way that it is rotatable about its own axis, and adjustable for position in the vertical direction, along its own longitudinal axis, and about a horizontal axis, so as to permit of forming helical, spur, relieved and other types of gear teeth.
The various movements accomplished by the tool must therefore be synchronized exactly with the movement of the work if correct tooth geometry of the gears in production is to be ensured.
Hitherto, almost all the manufacturers of gear cutting machinery have adopted similar expedients for the final stage of the aforementioned mechanical linkage (numerically controlled or otherwise), or at all events, of the worktable drive, at the point where high precision and reversibility of the drive are both essential requirements; such expedients are based generally on the use of one or two worm gear pairs, the final worm wheel being rigidly associated with the worktable (e.g. as in U.S. Pat. No. 3,232,169 and in U.S. Pat. No. 3,318,193).
Arrangements of this general type are beset by the drawbacks of excessive wear which occurs at high speed operation, and the rather mediocre quality of the average worm wheel, which is hob finished and keyed direct to the worktable, with the result that errors are also transmitted direct to the worktable. In addition, the precision demanded of conventional machines is often rendered unobtainable through the effect of overheating, which occurs mainly through excessive friction or inadequate cooling. A build-up of heat beyond certain limits will in fact lead to distortion of the components affected, which results ultimately in loss of precision.
These problems are tackled in U.S. Pat. No. 4,435,110 by adopting a hypoid gear pair, installed at the final stage of the linkage connecting with the worktable. In this patent system, the hypoid pair operates in association with two cylindrical gears associated rigidly with the worktable. The hypoid gears serve to change the direction of movement produced by the linkage and to reduce speed through the worktable drive line, whilst the cylindrical gears reduce the final speed of the worktable to suit the rotational speed of the hob.
With an arrangement such as this, one achieves a reduction in the manufacturing costs of gear cutting machinery, inasmuch as the hypoid gear drive is less expensive than drives utilizing worm gear pairs with worm wheel and worktable directly associated, and provides a wider range of speeds.
In addition to the train of gears described, the patent system in question also comprises a second train comprising a bevel gear pair and a cylindrical pinion; this second linkage is driven independently, the cylindrical pinion meshing with the cylindrical gear of the worktable, and serves to take up any backlash that may be created when rotation of the table is reversed.
A transmission such as this is complicated and costly, however, and difficulties in adjustment tend to compromise continuity of precision.
In other gear cutting machines, especially those where rotation of the worktable is controlled by a motor independent from that of the hob, though with a direct connection effected between the two motors, e.g. through a numerical control, use is made of two cylindrical gear pairs with parallel axes (pinion and wheel) in order to obtain a sharp reduction in the speed of the worktable, which turns as one with the wheel of the final pair.
In this type of transmission, one also has a second train that is driven from the wheel of the first pair and drives the wheel associated with the worktable.
Whilst it is true that one obtains a good degree of precision and a good speed range in this fashion, the drive is rendered cumbersome. More exactly, the driving pinion is keyed direct to the shaft of the motor, and as the worktable is vertically disposed and parallel gears are adopted, the motor must also be mounted vertically; this signifies bulk, reduced access, as the motor is housed in the casting of the machine frame, insufficient heat exchange, leading to hot spots at the work areas, and difficulties in servicing the machine.
Accordingly, the object of the invention is to embody a simply, inexpensive drive system for the worktable of an automatic gear cutting machine, from which the aforementioned drawbacks are absent.